1. Field of the Invention
The present invention relates to a power supply using a DC/DC converter and, more particularly, to a direct current source circuit in which a load share of each converter is easily set and the construction of the circuit is simplified.
2. Description of the Background Art
For a power supply such as a PDP (plasma display panel) TV, a large scale DC/DC converter is used. Hereinafter, the DC/DC converter is referred to as a converter. Since the above PDP TV is thin generally (about 100 mm), the power supply to be installed at the PDP TV must be thinned.
FIG. 1 is a view showing the construction of a source circuit for a conventional PDP TV. Rectifiers 12 and 13 are connected to rear ends of a plug 11. A standby power source 14 constructed by the converter is connected to a rear end of the rectifier 12 and outputs 5.6V. A power-factor control circuit 15 and a condenser 16 are connected to rear ends of the rectifier 13, and panel power sources 17-19 defining constructed by the converter are connected to rear ends of the power-factor control circuit 15 and the condenser 16.
A V.sub.s power source 17 is used for panel sustaining and generates outputs of -160.about.-190V, 1.9 A. A V.sub.D power source 18 is used for panel addressing and generates outputs of 50.about.90V, 1.7 A. V.sub.cc power source 19 is used for controlling a logic unit and generates outputs of 5V, 6 A. A signal processing power supply 20 is a power source for use with a TV/panel interface.
FIG. 2 shows the construction of a circuit for a converter constituting power sources 16 though 20, and FIG. 3 shows the waveform of an operation of a converter circuit.
A converter illustrated in FIG. 2 is a conventional current resonance converter. Transistors Q.sub.1 and Q.sub.2 constituting a main switch are connected to a DC power source V.sub.in in serial. Current diodes D.sub.1 and D.sub.2 are connected respectively to the transistors Q.sub.1 and Q.sub.2 in parallel. The transistors Q.sub.1 and Q.sub.2 are turned on and off in turns, with each being provided with control signals V.sub.gs1 and V.sub.gs2 by a feedback control circuit non-illustrated.
A smoothing condenser C.sub.v, a first side coil NP of a transformer T.sub.1 (converter transformer), and a serial circuit of a resonance condenser C.sub.i are connected to the transistor Q.sub.2 in parallel. The transformer T.sub.1 steps up or down a voltage and separates first and second sides electrically. Since the first side coil N.sub.p of the transformer T.sub.1 has a coil structure of loose coupling, inductance components L.sub.r and L.sub.p are formed by a leakage inductance. The inductance components L.sub.r and L.sub.p and the condenser C.sub.i constructs a serial resonance circuit. A second side coil N.sub.s of the transformer T.sub.2 and diodes D.sub.3 and D.sub.4 constructs a rectifier circuit. An output voltage V.sub.out is supplied to a load through a smoothing condenser C.sub.0.
The operation of the converter in FIG. 2 will be described in brief using a waveform view of FIG. 3.
When the signals V.sub.gs1 and V.sub.gs2 are supplied to the transistors Q.sub.1 and Q.sub.2 by the non-illustrated control circuit, the transistors Q.sub.1 and Q.sub.2 are turned on/off in turn. In addition there are dead time periods t.sub.3.about.t.sub.4 and t.sub.7.about.t.sub.8 during which both transistors Q.sub.1 and Q.sub.2 are turned off at the same time. In FIG. 3, applied voltages of the transistors Q.sub.1 and Q.sub.2 are represented as square waves V.sub.ds1 and V.sub.ds2, and currents thereof are represented as I.sub.d1 and I.sub.d2.
When the transistors Q.sub.1 and Q.sub.2 are turned on/off by turns at a certain particular switching frequency, a square wave voltage V.sub.ds2 is applied to the serial resonance circuit constructed of the inductance components L.sub.r and L.sub.p and the condenser C.sub.i and a resonance current I.sub.r flows in the serial resonance circuit.
FIG. 4 is a view showing the frequency characteristics of a serial resonance circuit in a converter. In the serial resonance circuit constructed of the inductance components L.sub.r and L.sub.p and the condenser C.sub.i, the resonance current varies according to the change of the frequency. By setting the range of frequency control as f.sub.1.about.f.sub.2, which are greater than the resonance point f.sub.0, the resonance current I.sub.r varies according to the switching frequency of the transistors Q.sub.1 and Q.sub.2, and it is increased when the frequency is decreased and it is decreased when the frequency is increased.
As illustrated in FIGS. 2 and 3, the resonance condenser C.sub.i is charged and discharged by the resonance current I.sub.r and thus the voltage V.sub.ci is changed. In addition, in the dead time periods during which the transistors Q.sub.1 and Q.sub.2 are turned off at the same time, a current I.sub.cv is supplied from the smoothing condenser C.sub.v to the serial resonance circuit. When the currents I.sub.d1 and I.sub.d2 flowing in the transistors Q.sub.1 and Q.sub.2 are controlled as I.sub.d &gt;0, current flows in the current diodes D.sub.1 and D.sub.2 and zero voltage switching or zero current switching is implemented.
In a case where the above-described converter is applied to a PDP TV, the converter constituting a power unit is required to have a limitation on height for the purpose of thinning the PDP TV. To satisfy the above requirement, a large scale converter can be constructed by connecting a plurality of small scale converters with respect to the large scale power source such as the V.sub.s power source 17 of FIG. 1.
As seen from above, one problem is that a load of each converter, that is, current flowing in each converter transformer has to have an uniform amount. Another problem is that a control circuit is required to be installed at each converter, thus increasing the cost.